Awesome work analysing and solving deeper issues in base design, things that just haven't really changed - see: indar revamp in progress.
I absolutely 100% agree that a fight over terrain between bases should be allowed to flow back and forth. Defenders need time to pull vehicles before attackers are at their doorstep. Both teams' base turrets' field of view should not overlap one another (let alone reach the opposing base! My god that's awful) allowing a neutral, fair space in the middle for a fight to take place that gradually helps out whichever side begins to lose as they get pushed back into coverage of their turrets.
Now, here's where I think something was overlooked. Removing the worst bases as they are doing, the ones that can't easily be reworked or block a road, that's good. Removing more because they are too close to one another and for the sake of numbers overlooks a clear opportunity.
Bases currently are insanely small for the number of players participating. I know I've said this many times, sorry for those that have read it. PS2's population has gone through the roof compared to your average shooter, yet the base design is actually smaller. There's usually one objective point, in a single room, for 48 vs 48 and above to fight over. That is atrocious and inexcusable.
So what to do? Well ideally, yeah you would have fewer bases with much much larger gaps between. Is that scale of a revamp possible at the moment? Not really. Luckily, there is a damned faster route that accomplishes a lot of the benefits in one swoop:
Combine several bases into one large capturable territory. All bases are simultaneously contestable, objective points spread out between them. Territories are now linked by lattice.
Immediately you are fighting over more buildings and CPs, thinking and fighting on the scale that PS2 should be. Bonus: ground vehicles are not 100% irrelevant and locked out behind walls or raised bases. They can participate yet can't directly farm walled bases, only interdict movements to new objective points / bases.
On the matter of spawn camping, again I think you've got to the heart of the issue and gone to the necessary lengths to solve it, rather than the minor tweaks we see at most bases. It feels like developers are only in the game of mitigating the farm rather than preventing it. I wonder whether that's a top down decision that ruled out the necessary changes, a lack of time, or have they not yet reached the same conclusion?
In any case while I think those beautiful diagrams of spawn tunnels gets far, far closer to the mark than the developers admit (and as players have said since the beginning referencing PS1), I don't think they are ideal.
What you are asking for here is nothing more than a space where infantry can fight out of the spawn room where vehicles do not interfere. There is nothing special about tunnels. All you need is a building so big that you can put the spawn at one end and the objective point (or SCU) at the other and have a fight between.
The developers insist on "bigger spawn room = better". Yeah, sure, spreading out the exits has a very marginal improvement as enemy infantry struggle to cover multiple exits. Except the number of players in PS2 means they still cover all exits and aircraft were never affected anyway.
The obvious truth is that all you're doing is shifting the position where players are inevitability getting farmed when they exit the safety of those shields, rather than any fundamental or far reaching attempt to prevent said farming. I.E. a roof and walls AFTER the shields to cut out vehicles and cover that favors defenders.
What would a good spawn room look like? Basically take the Hossin warehouse building, put two small spawn rooms in adjacent corners and an SCU / objective point at the other end. Make it heavily favor defenders so even enemy infantry have a hard time camping. For example, give defenders a balcony only on their end.
And of course not forgetting that's only the primary spawn, there should really be at least two unshielded buildings (shy of warehouse size though) in completely different locations housing small shielded teleporter rooms.
As for getting between buildings there is a lot of merit in tunnels linking buildings. I don't mean to understate that because it absolutely is a direction I wish Daybreak would use in every base, beyond underground bases. Raised, enclosed skyways would do this with the added benefit of allowing roads to pass underneath.
That said, it's the principle that I advocate. In practice, the exact opposite of what we want for good quality infantry fighting is is one long, narrow tunnel that would devolve into a grind fest. It should be wide, it could have two floors, there could easily be two between large buildings, and the buildings should be as close as possible. That would be the "tunnel option".
Alternatively, link buildings via entire rooms themselves. More of a building complex where you never really step into corridors. Scale is everything here. You can go outside for another approach but you the point is you don't have to.
Also, anything that mimics the infantry centric goal of tunnels, like some areas on Hossin where a tree provides cover from aircraft while walls, buildings and terrain block tanks, are entirely commendable and satisfactory too. It's just disappointing the entire bases there are raised such that ground vehicles are 100% locked out anyway so they can't even have minimum impact.
However, I would not go as far as to make this standard between literally every building. That's overkill. It's just a tool, one to help defenders reach an objective or two but after that it has served the purpose of breaking the spawn camp.
Exiting those buildings to get to the last points should involve vehicles to give them a role and to create a sliding scale of difficulty - attackers initially have vehicle support but not towards the end. Also to break the enclosed nature, giving infantry more space to think and play, with cover of course.
Still, ground vehicles near an objective (as opposed to those interdicting between bases / clumps of buildings) should operate on the basis of risk : reward, getting much closer and having limited mobility like tanks truly would in urban areas.